The effect of bovine brain calmodulin on the assembly of pure bovine brain tubulin has been examined in the presence and absence of microtubule-associated proteins (MAPs). In the absence of MAPs, calmodulin enhances the rate and extent of polymerization of pure tubulin, probably by sequestering Ca2+ from tubulin since the effect is mimicked by ethylene glycol bis(beta-aminoethyl ether)N,N,N',N'-tetraacetic acid and parvalbumin. From stoichiometric considerations, all 4 Ca2+ binding sites of calmodulin appear to participate in this effect. In the presence of MAPs, calmodulin confers increased Ca2+ sensitivity on the tubulin polymerization process, enhancing the inhibitory effect of Ca2+ on the rate and extent of assembly. The effect of calmodulin on the assembly of tubulin is dependent on the presence of Ca2+. The data suggest that calmodulin of both low (Ca1-22+.calmodulin) and high (Ca3-42+.calmodulin) Ca2+-induced inhibition of polymerization. Thus, calmodulin has dual and opposing actions on Ca2+ sensitivity of tubulin polymerization depending on the presence or absence of MAPs.